Fix event targeting for overlay scrollbar thumbs (in native UI).

Fix event targeting for overlay scrollbar thumbs (in native UI).

Views with layers are targeted based on their layer's transform rather
than their local coordinates. See View::GetTransform(). Hence the thumb
did not receive events until the translated area was hovered. To resolve
this, make the thumb 11 + 4 = 15 dp wide all the time, and just slide
back and forth so that 4dp is off the left or 4dp is off the right of
the 11dp track, making sure the thumb always covers the whole track
(it won't extend beyond visually or in terms of event handling because
of the track's layer clipping).

BUG=666798
Review-Url: https://codereview.chromium.org/2520433003
Cr-Commit-Position: refs/heads/master@{#433664}
(cherry picked from commit 03722d39ae28885ebd6de91429325776c77da2a7)

Committed: https://chromium.googlesource.com/chromium/src/+/045e98625ab0fed012c6fdfd7fb13c9194d43c8e

[tdanderson, 2016-11-28 19:45:12]

Description was changed from

==========
Fix event targeting for overlay scrollbar thumbs (in native UI).

Views with layers are targeted based on their layer's transform rather
than their local coordinates. See View::GetTransform(). Hence the thumb
did not receive events until the translated area was hovered. To resolve
this, make the thumb 11 + 4 = 15 dp wide all the time, and just slide
back and forth so that 4dp is off the left or 4dp is off the right of
the 11dp track, making sure the thumb always covers the whole track
(it won't extend beyond visually or in terms of event handling because
of the track's layer clipping).

BUG=666798

Review-Url: https://codereview.chromium.org/2520433003
Cr-Commit-Position: refs/heads/master@{#433664}
(cherry picked from commit 03722d39ae28885ebd6de91429325776c77da2a7)
==========

to

==========
Fix event targeting for overlay scrollbar thumbs (in native UI).

Views with layers are targeted based on their layer's transform rather
than their local coordinates. See View::GetTransform(). Hence the thumb
did not receive events until the translated area was hovered. To resolve
this, make the thumb 11 + 4 = 15 dp wide all the time, and just slide
back and forth so that 4dp is off the left or 4dp is off the right of
the 11dp track, making sure the thumb always covers the whole track
(it won't extend beyond visually or in terms of event handling because
of the track's layer clipping).

BUG=666798

Review-Url: https://codereview.chromium.org/2520433003
Cr-Commit-Position: refs/heads/master@{#433664}
(cherry picked from commit 03722d39ae28885ebd6de91429325776c77da2a7)

Committed:
https://chromium.googlesource.com/chromium/src/+/045e98625ab0fed012c6fdfd7fb1...
==========

[tdanderson, 2016-11-28 19:45:13]

Committed patchset #1 (id:1) manually as
045e98625ab0fed012c6fdfd7fb13c9194d43c8e (tree was closed).